Seat of second party (8)
I believe the answer is:
backside
'seat' is the definition.
('backside' can be a synonym of 'seat')
'second party' is the wordplay.
'second' becomes 'back' (to second a motion is to back it).
'party' becomes 'side' (I've seen this before).
'back'+'side'='BACKSIDE'
'of' acts as a link.
(Other definitions for backside that I've seen before include "Buttocks" , "Rear" , "Bottom" , "Behind" , "Hind part" .)